This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-258621 filed on Aug. 29, 2000.
1. Field of the Invention
The present invention relates to an electrically-driven power steering system for assisting a steering force of a steering wheel by using a motor.
2. Related Art
U.S. Pat. No. 6,032,756 (JP-A-9-315330) discloses a conventional electrically-driven power steering system, which includes an interlock means for permitting and forbidding a motor to rotate. In the conventional system, when a manipulating direction of a steering wheel is different from a driving direction of the motor in case of a failure of a central processing unit (CPU) for controlling the motor, the motor is forbidden to rotate by the interlock means.
In the above conventional system, although the CPU failure can be detected by the interlock means, it is hard to detect the CPU failure when the steering torque (torque sensor signal) enters a dead-zone of the interlock means where the motor is permitted to rotate.
The present invention has an object to provide an electrically-driven power steering system where a failure of a motor control unit, that is, abnormality in a command value of the CPU, can be surely detected.
In the present invention, an electrically-driven power steering system comprises a command value check circuit for checking whether the current command value calculated by the motor control unit is normal or abnormal with respect to a signal from a torque sensor, and a drive signal check circuit for checking whether the drive signal outputted from the motor control unit is normal or abnormal with respect to the current command value.
According to this construction, when the motor control unit fails, it can be checked whether its failure is an abnormality of the current command value or of the drive signal. The failure of the motor control unit can be surely detected by checking both of the current command value calculated in accordance with the steering torque and the drive signal set based on the current command value.
When the calculated current command value is within a suitable range, the command value check circuit determines the current command value to be normal with respect to the signal from the torque sensor. When the current command value is outside the suitable range, the command value check circuit determines the current command value to be abnormal. In the same manner, the drive signal check circuit determines the drive signal to be normal with respect to the calculated current command value when the drive signal is within a suitable rage, and to be abnormal when the drive signal is outside the suitable range.
The electrically-driven power steering system further comprises a system stop circuit for stopping any one of the motor control unit and the motor drive circuit, when the current command value or the drive signal is determined to be abnormal by the command value check circuit and the abnormality state continues for a predetermined time.